The purpose of this section is to document current scientific knowledge concerning the
risks of listeriosis in relation to fishery products, in order to enable identification of
risk contributing and risk mitigating factors for facilitating risk management decisions.
While the risk assessment includes some quantitative information, the time frame of the
Consultation did not permit a quantification of the risks of listeriosis associated with
fishery products.

L. monocytogenes is a bacterial pathogen causing serious human illness such as
perinatal infections, septicaemia and meningitis. More recently, a new form of disease
caused by L.monocytogenes has been recognized involving mild
gastrointestinal symptoms. Although listeriosis occurs infrequently, at an annual
incidence rate of 2 to 10 cases per million, the fatality rate is high, usually in the
range of 20-30%. Highly susceptible individuals include pregnant women, neonates, elderly
people and immunocompromised individuals. This last group includes, in decreasing order of
risk, organ-transplant recipients, patients with AIDS, HIV-infected patients and patients
with cancer. We are likely to see an increase in the incidence of listeriosis as the
numbers of susceptible individuals and vulnerable groups increase over the next decades.

Although other routes of transmission have been described, indistinguishable strains
have been isolated from epidemic cases and from the food implicated, clearly identifying
the role of food in the epidemiology of listeriosis. Foods associated with transmission
are characteristically highly processed, have extended shelf lives at refrigeration
temperatures, are capable of supporting the growth of L. monocytogenes
and are consumed without further cooking. Although some strains of L. monocytogenes
are more frequently associated with human diseases, at this time, all strains must be
considered to be potentially pathogenic.

Listeriosis is mainly reported in industrialized countries with few or no reports from
Africa, Asia and Latin America. It is not known whether this reflects different exposure
rates, consumption patterns, dietary habits, host susceptibility, or lack of testing
facilities in different regions.

4.3.2.1 Water and raw fish

Unpolluted seawater and ground waters used in aquaculture are generally free from this
organism, and fish from these environments are uncontaminated. In temperate regions, the
organism has been isolated from surface waters and lakes, and in coastal waters subject to
pollution or contamination from industrial, human or animal sources. When L. monocytogenes
is present in fish from these environments it is usually present in very low numbers. In
tropical regions, the reported incidence of L. monocytogenes is very low and
freshly harvested fish from these environments are generally free of this pathogen.

Different strains of L. monocytogenes may be isolated from fish raw material and
from final products suggesting that fish can be contaminated at any point between harvest
and consumption. In particular, L. monocytogenes can colonize the processing
environment and this has been established as a primary mechanism of contamination for some
products.

4.3.2.2 Cold smoked fish

Cold smoked fish is produced by filleting raw fish which is subsequently salted
(3.0-3.5% NaCl in water phase), dried and smoked at a temperature < 30oC.
The fish is then usually sliced and vacuum packaged. It should be stored at a temperature
£ 5oC, and under these conditions has a recommended shelf-life of 3 to 6
weeks. Several surveys reveal that 10-60% of freshly processed cold smoked salmon is
contaminated with L. monocytogenes. The contamination level is usually £ 100
cfu g-1. A similar contamination rate has also been observed on other lightly
preserved fish and fishery products that are processed without a listericidal step. During
storage for 2 to 3 weeks at 5oC L. monocytogenes may increase in
numbers but it is difficult to predict the magnitude of increase. Uncontrollable factors
including the physiological state of the organism and the presence and type of microflora
on the product will affect the potential for growth of L. monocytogenes.

The processing environment contributes to the presence of L. monocytogenes
in cold smoked fish. L. monocytogenes can become established on processing
surfaces and may be very difficult to detect and to remove. After production, time and
temperature of storage are the most important factors that determine outgrowth. Survey
data suggest that levels rarely exceed 103 cfu g-1 at the time of
consumption.

4.3.2.3 Cooked fishery products

L. monocytogenes has been recovered from a range of ready-to-eat fishery
products, such as cooked shrimps and cooked crabmeat. The rate of contamination of cooked
ready-to-eat products varies. For example, less than 1% of ready-to-eat fishery products
imported into Canada during 1996/98 were contaminated with L. monocytogenes
while pooled data from 6 other published reports (involving 6 countries, 498 samples) on
ready-to-eat cooked shrimp indicated a contamination rate of about 4.8%. This indicates
post-process contamination, as L. monocytogenes should be reduced to undetectable
levels by cooking. When cooked products are stored at chill temperatures (i.e. < 5°C)
for extended periods, the potential for growth of L. monocytogenes will be
less restricted because of the absence of competitive flora.

4.3.3.1 Growth potential

The major factors controlling the fate of microbial populations in fish and fishery
products are temperature, water activity and pH. Specific organic acids, and many
preservative compounds, may also play an important role in reducing microbial growth in
fishery products. Thus, the potential for growth of Listeria in fishery products is
related to the product composition, type and intended end use. Some examples are shown
below.

Products with the potential to contain high levels of L. monocytogenes at the
time of consumption

Lightly preserved fish products (pH >5.0, NaCl < 8.0%), including:

lightly salted

cold smoked

"gravad"

marinated

fermented

caviar

Heat-treated, before packaging:

hot smoked fish

cooked shrimp

reconstituted fish protein products

Products unlikely to contain high levels of L. monocytogenes at the time of
consumption

The tolerance to a particular environmental constraint is greatest when all other
conditions are optimal for growth. For L. monocytogenes, growth is maximal
when temperature is in the range 20  25°C, although the growth rate is fastest at
~37°C. When several factors are sub-optimal for growth, the growth permitting ranges of
each of those factors will be reduced. Thus, growth at the lowest pH or water activity
will only occur when all other conditions are optimal. This behaviour has been embodied in
the "Hurdle Concept" and is reflected in the product categories presented
earlier in this section.

There are exceptions to this general trend, e.g. while slightly elevated salt
concentration may inhibit growth rate, it may at the same time increase tolerance to high
temperature of many bacterial species.

Table 1: Growth limits for L. monocytogenes

Environmental Factor

Lower Limit

Upper Limit

Temperature (°C)

-0.3 to + 3

~ 45

Salt (% NaCl, water phase)

< 0.5

12-13

Water activity (aw)*

0.91  0.93

> 0.997

pH (HCl as acidulant)

4.2  4.3

9.4  9.5

Lactic acid (water phase)

0

3.8  4.6 mM, MIC of undissociated acid

(800 
1000 mM, MIC of sodium lactate)

* values for NaCl as the humectant

4.3.3.2 Growth rates

The physico-chemical attributes of many fish and fishery products will permit the
growth of L. monocytogenes. Some representative growth rates for conditions
relevant to these products are shown in Table 2.

As discussed earlier, it is recognized that other factors may affect potential for
growth of L. monocytogenes in foods. Also, L. monocytogenes is known to
be sensitive to quaternary ammonium compounds, chlorine and sanitizers containing
peracetic acid and peroctanoic acid. Irradiation can effectively reduce L.
monocytogenes to undetectable levels in products. Due to all these factors, the
presently available predictive models for growth are unable to make accurate predictions.
Accurate prediction of growth is a necessary tool for a proper risk assessment.

The epidemiological pattern of human listeriosis is a background of sporadic cases with
occasional outbreaks. A decrease in the number of sporadic cases has been observed during
the last ten years in the UK, USA and France. While several reports indicate that fish and
fishery products can be frequently contaminated with L. monocytogenes, no major
outbreaks associated with these products have been reported. This may be due to inadequate
surveillance systems in several countries or because not all factors contributing to both
sporadic cases and outbreaks associated with fisheries products have been identified. A
low number of cases (Table 3) have been linked to fish associated listeriosis outbreaks
when compared to listeriosis outbreaks associated with other foods.

Invasive listeriosis includes perinatal infections, central nervous system infections
and bacteremia. Neurologic sequelae have been observed in 30% of patients with central
nervous system infections. Ninety per cent of cases occur in populations with impaired
immunity. The incubation period ranges from 1 to 90 days, with a mean of 3 to 4 weeks.
This variability may reflect variation in the number of cells ingested, differences in
host susceptibility, or differences in virulence of the strains. Excepting nosocomial
infections, person-to-person transmission has not yet been firmly documented during
food-borne community acquired outbreaks. The role of healthy carriers (4 - 6% of the
healthy population) in the epidemiology of listeriosis warrants further studies.

The dose response relationship of L. monocytogenes for humans is not known. In
general, the infectious dose of a foodborne pathogen depends on a number of variables
including the condition of the host, the virulence of the strain, the type and amount of
food consumed, and the concentration of the pathogen in the food. Animal studies suggest
that reducing levels of exposure will reduce clinical disease. From the reported numbers
of Listeria in contaminated food responsible for epidemic and sporadic foodborne
cases, there is little evidence that a very low number of L. monocytogenes in food
causes listeriosis. In a number of outbreaks, enumeration of units of implicated foods
indicated both high (> 1,000 cfu g-1) and low levels (< 100 cfu g-1)
of contamination. Some of these data related to fish and fishery products
are summarized in Table 3. High levels of L. monocytogenes (103107 cfu gr1)
have been detected in soft cheeses involved in 5 outbreaks. These facts, together with
data on the recovery of the organism from implicated foods suggest the likelihood that
high infective doses are involved in most cases.

From the foregoing discussion, it is clear that ready-to-eat fishery products
frequently contain L. monocytogenes. The level of contamination at the time of
consumption cannot be calculated with confidence. Usually low levels are found and it is
known that levels of ~10  100 cfu g-1 may occur infrequently at the time
of manufacture of cold smoked salmon. In cooked products, lower levels of contamination
occur. Growth of L. monocytogenes on these products can be demonstrated, although
growth is slow under proper storage conditions. Calculations suggest that per capita
human exposure to doses of L. monocytogenes exceeding 1,000 cfu (total ingested
dose) is likely to occur several times each year. Despite this exposure, the total
incidence of invasive listeriosis is estimated to be 2-10 per million population per annum
in countries where data are available.

Listeriosis outbreaks may extend over many months or years, and with low attack rates.
The incubation period is usually of the order of 34 weeks, but may extend to over
three months. These factors combine to make outbreaks difficult to recognize and to
investigate. The severity of consequences of listeriosis is high, with 20 - 30%
case-fatality rates.

Recognizing the potential for fishery products to be a vehicle for large outbreaks of
listeriosis, it is prudent to seek appropriate strategies to minimize human exposure to
infectious doses of the organism. Currently, however, there is insufficient data for the
dose-response relationship to be determined, but there is growing consensus that the
majority of the healthy population is highly resistant to doses in the order of 1,000 to
10,000 cfu L. monocytogenes. There is no consensus on the dose required to cause
illness in susceptible individuals. However, using the available epidemiological data, and
consumption patterns, crude estimates of the risk of listeriosis from fishery products can
be made.

Regarding fish and fishery products, the dose-response has been estimated elsewhere
combining data on the incidence of listeriosis in Germany with data on the levels of L. monocytogenes
in smoked-fish in that country. It was necessary to make many assumptions, but all were
chosen to result in conservative estimates. Epidemiological and food survey data were
combined, using a predictive modelling approach, to estimate a dose-response relationship
for L. monocytogenes levels and incidence of listeriosis. Two methods were
used to model and calculate the dose-response relationship. Both methods gave a similar
result. Using that approach, for the estimated immunocompromised sub-population of Germany
(20% of the population), the model predicts a 1 in 59 million chance of infection from
consumption of a 50g serving of fish containing 100 bacteria per gram.

It must be emphasized that this estimate rests on many assumptions. Nevertheless, when
extrapolated across the population and the number of salmon meals consumed per year, the
estimate reflects the low incidence of listeriosis in those nations where epidemiological
data are available. The above estimate, is of the same order of magnitude as the reported
per annum incidence of listeriosis.

Thus, despite the fact that there is considerable potential for fish and fishery
products to cause listeriosis, the available data indicate that this potential has not
been observed. There is little epidemiological evidence to implicate fishery products in
large outbreaks. The reasons for this are unknown.